In the AMBER community, these trajectories are often saved with the
suffix ‘.crd’. This extension conflicts with the CHARMM CRD
format and MDAnalysis will not correctly autodetect AMBER “.crd”
trajectories. Instead, explicitly provide the format="TRJ"
argument to Universe:

trajectory; the reader always assumes that these are orthorhombic
boxes.
* The trajectory does not contain time information so we simply set
the time step to 1 ps (or the user could provide it with the dt argument)
* Trajectories with fewer than 4 atoms probably fail to be read (BUG).
* If the trajectory contains exactly one atom then it is always
assumed to be non-periodic (for technical reasons).
* Velocities are currently not supported as ASCII trajectories.